Recently, there has actively been developed a writable write once read many (hereinafter, referred to as "WORM") type optical disc which is capable of being reproduced by means of an ordinary CD player with reference to standards called as "Orange Book" and "Red Book". In these standards, with respect to a tracking error signal (hereinafter, referred to as "TE") to be used for the above WORM type optical disc, only a data standard for a push-pull method using one beam is present.
In the CD players used as present, however, the TE control is mainly effected by use of a so-called "three-beams method", or "time difference method" using one beam. Accordingly, some optical discs only have a poor playability. More specifically, some discs can assuredly provide the TE based on the push-pull method but little provide the TE based on the three-beams method. Such cases inevitably cause a confusion in the user side, and therefore it is desired to adopt an active measure for avoiding such a confusion.
In order to need the above requirement, it is conceivable to adopt a method wherein a TE based on the three-beams method is defined. In the three-beams method, however, the TE based on the three-beams method cannot be specified because (1) the angle .theta. formed between a line connecting two subordinate beams and the track has been designed in various manners, (2) if the trace of the movement of a pick up is not provided precisely, the above angle .theta. may be changed during the movement, etc.